You should name the ranges that you want to sum together by selecting the range of cells and create a name in the Name box in the top left corner. e.g "total1"
You can then use this name to create a formula like "=SUM(total1)".
If you are adding columns it is best to make the named range with one empty column at the end. This way, inserting new columns will update the named range reference to update the formula result.
For non consecutive columns you should be able to update the first row formula and then autofill or copy for the rest of the columns.
